The present tool was developed for making a conical undercut in the walls of a pre-formed hole, which hole then receives an expanding anchor bolt which is designed to expand into and substantially fill the conical undercut. This structure is particularly useful in setting bolts into an existing concrete, stone, or other masonry structure where the hole is drilled after the structure is built, as distinguished from a wall having bolts embedded therein before the concrete sets. The undercutting of the hole greatly increases the reliability of the anchor bolt, as compared with an anchor bolt set into a hole having only cylindrical walls and relying on friction to prevent withdrawal of the bolt after it has been expanded.
The prior art shows many patents for tools used to undercut holes. For instance U.S. Pat. Nos. 2,225,165 to Dunlap and 2,401,515 to Schutte et al show underreamers having wide blades pivoted to the body of the tool and extensible therefrom when the tool is pushed into a hole so that a pin moves in a diagonal slot in each cutter.
U.S. Pat. Nos. 2,433,976 to Babka, 3,021,773 to Cogsdill and 3,025,729 to Heuser show similar tools in which the cutter elements are pivoted midway of their lengths so that the outer portion of the cutter rotates outwardly around the pivot when the inner portion is cammed oppositely. These patents also show live abutments adjustable to locate the undercutting at a desired distance inside the hole, and further show spring returns operative to retract the cutters when axial thrust is released from the tool by the operator.
Copies of the above five patents were filled with this application in lieu of a Prior Art Statement.